1. Field of the Invention
The present invention relates to a toner for use in recording processes such as electrophotographic processes, electrostatic recording processes, electrostatic printing processes, and the like.
2. Description of the Related Art
To date, many electrophotographic recording processes are known. In a typical electrophotographic process, an electrical latent image is formed by a variety of methods on a member for carrying an electrostatic image, hereinafter simply “photosensitive member”, using a photoconductive material, and is developed into a visible toner image using a toner. The toner image is transferred onto a suitable recording medium, such as paper, and is then fixed on the recording medium by application of heat, pressure, or the like, to obtain a copy.
Examples of the methods for forming visible toner images from electrical latent images include cascade development; magnetic-brush development; pressure development; magnetic-brush development with a two-component developer containing a carrier and a toner; noncontact single-component development in which toner is transferred from a toner supporting member onto a photosensitive member without the photosensitive member making contact with the toner supporting member; contact single-component development in which a toner supporting member is pressed against a photosensitive member to transfer the toner by an electric field; and jumping development using a magnetic toner.
Recent technical trends require electrophotographic apparatuses, such as printers, to have higher resolutions as measured in dots per inch (dpi). The desired resolutions are now 1,200 dpi and 2,400 dpi, which are higher than the 300 dpi and 600 dpi conventionally required. Higher resolutions require finer development systems. Moreover, recent copying machines incorporate digital technology to achieve advanced functions. In particular, copying machines now use lasers to produce electrostatic images to achieve higher resolutions. As with printers, copy machines also require high-resolution, fine development systems.
Furthermore, the field of electrophotography has seen rapid development of color printing. Since color images are developed by adequately superimposing yellow, magenta, cyan, and black toners, toners are required to have characteristics suitable for such development (hereinafter referred to as “development characteristics”), which are different from those required in a single toner process. Accordingly, the electrification of the toners must be uniformly controlled.
In order to control the electrification of toners, charge control agents are conventionally used. In general, charge control agents can be roughly classified into two types, namely, (i) complex compounds having complex structures in which ligand components coordinate with central metals and (ii) polymer compounds containing polar functional groups that function as the charging sites. Complex compounds are crystalline and exhibit low compatibility with binder resins; accordingly, a toner production method must be carefully selected and controlled to uniformly disperse such complex compounds. In contrast, charge control agents of a polymer compound type, which are highly compatible with resins, can easily form homogeneous dispersions; accordingly, fewer limitations are imposed on the process using this type of agent. An example of the polymer compound charge control agent is a resin containing a polymerizable polymer of a particular structure. For example, Japanese Patent Laid-Open No. 63-184762 discloses such a polymer compound charge control agent.
In an electrophotographic process, a toner image produced on a photosensitive member by development is transferred onto a recording member in a transfer step. The remaining toner in the image area and the fogging toner in the non-image area on the photosensitive member are removed in a cleaning step and stored in a waste toner storage. In a conventional cleaning step, a blade, a fur-brush, a roller, or the like has been used. These components require a large space and prevent size reduction of the apparatuses. Moreover, from the standpoint of ecology, a system with less waste toner and a toner having high transfer efficiency while causing less fogging are desired.
The transfer efficiency is known to decrease due to degradation in releasability of the toner from the photosensitive drum. The degradation occurs when the circularity or sphericity of the toner is low because a toner with low circularity or sphericity increases the area of contact between the toner and the photosensitive drum. Moreover, since the surface of such a toner has large irregularities, charges concentrate on edges and the so-called image force at the locations corresponding to these edges increases as a result.
The process of achieving high toner circularity differs depending on the method for making the toner. Methods for making commercial toners can be roughly classified into pulverization methods and polymerization methods. In pulverization methods, a binder resin, a coloring agent, and the like are thoroughly mixed by melting to obtain a homogeneous mixture. The mixture is then pulverized in a fine grinding mill and classified with a classifier to obtain a toner having a predetermined particle diameter. The toner obtained by the pulverization methods has irregularities in the surface since the surface has fractures resulting from milling. Accordingly, an additional process, such as applying mechanical impact, heat, or the like, is necessary to improve the surface quality and to achieve sufficiently high circularity.
Polymerization methods can be classified into two types, namely, association/aggregation methods and suspension polymerization methods. In the association/aggregation method, resin particles, a coloring agent, a releasing agent, and the like are associated or aggregated into particles of a predetermined diameter in an aqueous medium containing emulsion-polymerized resin particles as the binder resin component. In the suspension polymerization method, a polymerizable monomer composition containing a coloring agent, a releasing agent, a polymerization initiator and the like dispersed or dissolved in a polymerizable monomer (binder resin component) is prepared. The polymerizable monomer composition is then placed in an aqueous medium, formed into droplets of a predetermined diameter by application of shear force, and is suspension-polymerized to provide a toner.
The toner prepared by the association/aggregation method also has irregularities on the surface; thus, an additional process of heating the toner, adding another polymerizable monomer composition to perform seed polymerization, or the like is necessary to improve the surface quality. The toner prepared by suspension polymerization methods has fewer irregularities and is more spherical compared to other toners since the toner is polymerized in droplets. No additional process is required to achieve high circularity. An example of this type of toner is disclosed in Japanese Patent Laid-Open No. 2001-343788. As is described above, a toner capable of uniform electrification and having high transfer efficiency can be prepared by suspension polymerization using a charge control agent of a polymer compound type. An example of such a technique is disclosed in Japanese Patent Laid-Open No. 2000-056518.
Moreover, a toner can be stably and efficiently prepared by suspension polymerization using a water-insoluble inorganic salt as the dispersion stabilizer. Such a technique is disclosed in Japanese Patent Laid-Open No. 2002-108019.
As is described above, the transfer efficiency can be improved by increasing the circularity of the toner. However, some of the toner will remain on the photosensitive member after the transfer step unless the transfer efficiency is 100%. Thus, a cleaning step for removing the remaining toner is necessary. In the cleaning step, a toner having high circularity and thus high flowability is difficult to remove since the toner can pass under the cleaning blade. Accordingly, when the toner has high charge, an image force operates between the image carrying member and the toner, and thus the toner becomes difficult to remove in the cleaning step.
On the other hand, when the toner has low charge, the toner tends to scatter into a development unit or the like, thereby contaminating the interior of the printer, copy machine, or the like. The contamination may cause image quality degradation, image contamination, and defects in the apparatus.
Thus, a highly circular toner prepared with a charge control agent of a polymer compound type rarely satisfies all of the properties required in development, charging, and cleaning.